Do I create each schema as a design or as a relational model in one design?

As a relational model in one design.

How can I refer to the view of the other schema in the Query Builder

You an define a schema for view you are creating. If objects used in view definition belong to another schema they are include with schema name in generated select statement. By default objects in the same schema (as the view) are not prefixed with schema name - if you want schema name to be included then check "Include schema name in Query" check box on view dialog.

thanks for the answer.
I arranged the schema's as a relational design. This works well but has the folowing disadvantages:
- you still can't refer the view from the other schema in the Query Builder
- since we have many schema's, opening a design takes a while
- closing the unneccesary relation designs when opening a design (uncheck the checkboxes) takes a while